Aug. 17, 2012 -- Early in this year's West Nile virus season, the death toll is at 29 and rising. There have been about 700 illnesses reported so far, more than 400 of them serious meningitis or encephalitis.
It's an unusually severe West Nile season -- and now there's new evidence that the virus itself may be unusually dangerous. The new threat: kidney disease years after infection.
Eight in 10 people infected with West Nile virus don't get sick. At least not right away. A new study finds that even in people who never had serious West Nile symptoms, the virus can burrow deep into the body. Years later, this persistent infection often leads to kidney disease that gets worse and worse over time.
As many as 9% of people who have mild or no initial symptoms may have persistent West Nile virus infection, says Baylor University West Nile expert Kristy O. Murray, PhD, DVM.
"Right now, we have seen people continually decline. We have no specific treatment for them to reverse what is happening," Murray says. "Will they eventually need dialysis? It will mean following them even longer to see if some stabilize."
In an NIH-funded study, Murray's team has been keeping track of about 200 people infected with West Nile infection over the last 10 years. About 40% of them now are showing signs of kidney disease and lasting West Nile virus infection.
Patients who survived the terrible symptoms of severe West Nile disease -- sometimes-paralyzing meningitis or encephalitis -- were most likely to suffer persistent infection. These patients also were most likely to have severe kidney damage.
But this is happening even to people who never had symptoms -- people who learned of their West Nile infection only when they were tested when donating blood.
It's an "important" finding, says William Schaffner, MD, professor and chair of preventive medicine at Vanderbilt University and president of the National Foundation for Infectious Diseases.
"This study suggests that West Nile virus infection not only can persist, but that like a termite it slowly and surely gnaws away at kidney function," he says.
West Nile virus arrived in the U.S. in 1999 and spread across the country in 2002-2003. Since then, an estimated 3 million people in the U.S. have been infected.
Even if only 5% ended up with kidney disease -- a much lower percentage than seen in the Murray study -- over time that would mean about 150,000 Americans have persistent infections and are at risk of kidney disease. And more people are being infected every year.
The persistent infection can't spread from person to person, Schaffner says.
"A person with persistent infection is not hazardous in the home or anywhere else," he says. "We get it from mosquitoes, and they get it from birds. In a person it is a dead-end infection and is not going to go anywhere else."
There's no vaccine against West Nile virus. Vaccines do exist for similar viruses spread by mosquitoes, making a West Nile vaccine at least technically possible. Tests of blood and cerebrospinal fluid (CSF) can detect acute infection. Murray says her lab has developed a test to detect virus in the urine.
There's no treatment for West Nile virus infection. Murray says that in animal studies, existing antiviral drugs appear to make the infection worse, not better.
Anyone who has had West Nile in the past should get annual checkups of kidney function, Schaffner and Murray suggest.
The CDC said it would not comment on the work of Murray, a former agency employee, for this story.
Murray and colleagues reported their findings in the online journal PLoS. Since they submitted their results, Murray says her team has confirmed persistent human West Nile infection by directly seeing live virus in urine with an electron microscope, a more sensitive test than she used in the original study.
With about 700 reported cases in 43 U.S. states as of the second week of August, the 2012 West Nile season is off to a fast start.
Case counts and deaths have not yet peaked, says infectious disease expert Jorge Parada, MD, MPH, professor of medicine at New Orleans' Loyola University and medical spokesman for the National Pest Management Association.
"The game is not over yet," Parada says. "We will see cases continue to rise over the foreseeable future."
That's no surprise to Dallas, Texas, where a West Nile health emergency has been declared and officials have resorted to aerial spraying to kill off mosquitoes. Parada says that although spraying does kill many adult mosquitoes, whether it's effective must be determined on a local case-by-case basis.
More than 80% of cases reported so far have been in Texas, Louisiana, Mississippi, Oklahoma, South Dakota, and California. But the virus is present everywhere there are mosquitoes, heat, and a little bit of water.
Other than protecting oneself against mosquito bites -- wearing effective insect repellent and/or wearing long sleeves and long pants despite hot weather -- Parada says people should look to their homes.
"One thing relatively simple that most just don't do is take a walk around their homes," he says. "If there is standing water -- there often is, in a pot or birdbath or old tire -- remember that mosquitoes can breed in tiny amounts of water so long as it remains there for a few days. And clean your gutters: There is standing water up there. Don't make your house a breeding ground for mosquitoes that carry West Nile."